| // Copyright 2018 Developers of the Rand project. |
| // Copyright 2017 Paul Dicker. |
| // Copyright 2014-2017 Melissa O'Neill and PCG Project contributors |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| //! PCG random number generators |
| |
| use core::fmt; |
| use rand_core::{RngCore, SeedableRng, Error, le, impls}; |
| #[cfg(feature="serde1")] use serde::{Serialize, Deserialize}; |
| |
| // This is the default multiplier used by PCG for 64-bit state. |
| const MULTIPLIER: u64 = 6364136223846793005; |
| |
| /// A PCG random number generator (XSH RR 64/32 (LCG) variant). |
| /// |
| /// Permuted Congruential Generator with 64-bit state, internal Linear |
| /// Congruential Generator, and 32-bit output via "xorshift high (bits), |
| /// random rotation" output function. |
| /// |
| /// This is a 64-bit LCG with explicitly chosen stream with the PCG-XSH-RR |
| /// output function. This combination is the standard `pcg32`. |
| /// |
| /// Despite the name, this implementation uses 16 bytes (128 bit) space |
| /// comprising 64 bits of state and 64 bits stream selector. These are both set |
| /// by `SeedableRng`, using a 128-bit seed. |
| #[derive(Clone)] |
| #[cfg_attr(feature="serde1", derive(Serialize,Deserialize))] |
| pub struct Lcg64Xsh32 { |
| state: u64, |
| increment: u64, |
| } |
| |
| /// `Lcg64Xsh32` is also officially known as `pcg32`. |
| pub type Pcg32 = Lcg64Xsh32; |
| |
| impl Lcg64Xsh32 { |
| /// Construct an instance compatible with PCG seed and stream. |
| /// |
| /// Note that PCG specifies default values for both parameters: |
| /// |
| /// - `state = 0xcafef00dd15ea5e5` |
| /// - `stream = 0xa02bdbf7bb3c0a7` |
| // Note: stream is 1442695040888963407u64 >> 1 |
| pub fn new(state: u64, stream: u64) -> Self { |
| // The increment must be odd, hence we discard one bit: |
| let increment = (stream << 1) | 1; |
| Lcg64Xsh32::from_state_incr(state, increment) |
| } |
| |
| #[inline] |
| fn from_state_incr(state: u64, increment: u64) -> Self { |
| let mut pcg = Lcg64Xsh32 { state, increment }; |
| // Move away from inital value: |
| pcg.state = pcg.state.wrapping_add(pcg.increment); |
| pcg.step(); |
| pcg |
| } |
| |
| #[inline] |
| fn step(&mut self) { |
| // prepare the LCG for the next round |
| self.state = self.state |
| .wrapping_mul(MULTIPLIER) |
| .wrapping_add(self.increment); |
| } |
| } |
| |
| // Custom Debug implementation that does not expose the internal state |
| impl fmt::Debug for Lcg64Xsh32 { |
| fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { |
| write!(f, "Lcg64Xsh32 {{}}") |
| } |
| } |
| |
| /// We use a single 127-bit seed to initialise the state and select a stream. |
| /// One `seed` bit (lowest bit of `seed[8]`) is ignored. |
| impl SeedableRng for Lcg64Xsh32 { |
| type Seed = [u8; 16]; |
| |
| fn from_seed(seed: Self::Seed) -> Self { |
| let mut seed_u64 = [0u64; 2]; |
| le::read_u64_into(&seed, &mut seed_u64); |
| |
| // The increment must be odd, hence we discard one bit: |
| Lcg64Xsh32::from_state_incr(seed_u64[0], seed_u64[1] | 1) |
| } |
| } |
| |
| impl RngCore for Lcg64Xsh32 { |
| #[inline] |
| fn next_u32(&mut self) -> u32 { |
| let state = self.state; |
| self.step(); |
| |
| // Output function XSH RR: xorshift high (bits), followed by a random rotate |
| // Constants are for 64-bit state, 32-bit output |
| const ROTATE: u32 = 59; // 64 - 5 |
| const XSHIFT: u32 = 18; // (5 + 32) / 2 |
| const SPARE: u32 = 27; // 64 - 32 - 5 |
| |
| let rot = (state >> ROTATE) as u32; |
| let xsh = (((state >> XSHIFT) ^ state) >> SPARE) as u32; |
| xsh.rotate_right(rot) |
| } |
| |
| #[inline] |
| fn next_u64(&mut self) -> u64 { |
| impls::next_u64_via_u32(self) |
| } |
| |
| #[inline] |
| fn fill_bytes(&mut self, dest: &mut [u8]) { |
| impls::fill_bytes_via_next(self, dest) |
| } |
| |
| #[inline] |
| fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> { |
| self.fill_bytes(dest); |
| Ok(()) |
| } |
| } |